Ya, but she had time to know something was happening, and the bellowing, a few seconds before the door went down, apparently was enough time to pick up a gun.
Hearing the noise doesn't mean she heard it clearly. I don't know too many 90+ year olds who have good hearing. It's not like she was 19 with great hearing, eyesight, and quick reflexes. Yes, she had time to pick up the gun but we don't know how accessable she kept it.
If she had a clue that there were druggies coming to the house, or if she was indeed dealing herself, then I could see where she would be skittish about a bunch of men hollering and kicking her door in. If she had reason to think they were druggies, she had reason to fear for her life. I'm giving her the benefit of the doubt.
If her hearing was bad, and that seems likely given her age, she heard a big commotion and picked up her gun off the dresser. When her door crashed in she knew someone was coming to kill her and she started shooting. Unfortunately it was probably a small caliber piece and she just shot in the invaders' direction. But if she had known to aim at the head, then it would be that she knew to expect cops in vests.